1. Technical Field
The present invention relates generally to the production of synthetic polymer fibers and more specifically to an apparatus and method for more efficiently heat setting the polymer fibers.
2. Background Information
Synthetic fibers for use in the manufacture of synthetic yarns are produced by a process called spinning, wherein polymeric material is extruded through small holes of a device called a spinneret to form filaments of semi-solid polymer that are subsequently solidified to form an endless polymeric filament. For example, polyethylene terephthalate (PET) fibers, a type of polyester, is formed by a process called "melt spinning" in which melted polymeric material is extruded and then solidified by cooling. Once produced, the synthetic filaments are gathered and transported longitudinally in a lengthwise co-extensive bundle commonly referred to as a "tow." Typically, after formation synthetic filaments are then drawn or stretched, heat set and crimped before being cut and baled.
A typical drawing process involves transporting multiple tows in a side-by-side relation through drawstands having a series of rollers operating at progressively greater driven speeds to exert a lengthwise stretching force on the travelling tows and their individual filaments. Drawing exerts a lengthwise force on the filaments that pulls molecular chains together and orients the chains along the filament axis. Typically, the drawing process is done in one or more steps and is often done at an elevated temperature, but usually less than approximately 100.degree. C. Drawing creates a stronger yam than would be made from undrawn synthetic filaments.
After drawing, synthetic filaments are then subjected to a heat setting process in order to stabilize the stretched fibers by crystallization of the polymer molecules under controlled tension. Effective heat setting of synthetic polymer filaments requires heating the filaments to a temperature of over 150.degree. C. and often to a temperature of approximately 200.degree. C. A calendering apparatus having a series of heated rolls about which the tow travels peripherally in a sinuous path is usually used to heat set synthetic polymer tows. But polymeric materials in general, and PET in particular, exhibit low thermal conductivity. Furthermore, the interstitial spaces between individual filaments in a tow comprising collectively numerous individual filaments exacerbate the difficulty of transferring heat throughout the thickness of a tow. Because calender rolls rely on conduction to transfer heat from the surface of the roll through the tow, and because only a portion of the tow filaments actually contact the calender rolls, heat penetrates very slowly through the thickness of the tow.
In order to promote more rapid heat transfer through a tow, it is known to construct calendering apparatus with sufficiently long cantilevered rolls to permit the spreading of the individual filaments of the tow in the form of a ribbon or band along the length of the roll. In fact, the length of a typical calender roll can exceed 1.5 meters, thus necessitating a very large calendering apparatus structure with mechanical bearings sufficiently massive to support the rolls and to resist the bending moments and deflective forces imposed by the tows of the size and density conventionally being processed.
After being heat set, synthetic polymeric filaments are usually cooled and transported through a crimper, such as a so-called stuffer box, to impart texture and bulk to the individual filaments before further processing such as drying, cutting, and baling. Because conventional tow crimping equipment requires a thicker, denser tow than the thinly spread towband for which the conventional calendering apparatus is intended, a disadvantage of the use of such calenders with long massive rolls is that an additional unit of equipment must be interposed between the calender structure and the crimper to condense and reform the tow into a thickness suitable for delivery to the crimper. Often, the crimping process is accomplished at an elevated temperature and typically around approximately 100.degree. C.
Various water-based sprays and aqueous emulsions are used throughout the drawing, heat setting and crimping processes described above. Aqueous emulsions may be used to facilitate such characteristics as tow cohesion, lubrication, and heat transfer and various water and steam sprays may be use to adjust the temperature of the tow. For example, a typical drawing process might include a pretension stand followed by two drawstands arranged such that the synthetic filaments travel through a predraw bath of water-based emulsion following the pretension stand; through a draw chest of warm water spray between the two drawstands; and through a second draw chest of steam spray after the second drawstand.
The conventional practice of processing synthetic fibers in a wet state before heat setting is problematic because the tow retains moisture and this moisture must be evaporated from the tow before the synthetic filaments can be effectively heat set. A saturated polyester tow can easily contain approximately 25% moisture by weight as it enters the heat setting calendering apparatus. This moisture, much of which is in the interstitial area between filaments in the tow, must be evaporated before it is possible to raise the filament temperature to the heat setting point. Notwithstanding the practice of spreading the filaments into bands contacting the heated calender rolls, a tremendous amount of energy is still required to evaporate moisture in the towband before heat setting can begin. In fact, it is estimated that approximately two thirds of the energy used by conventional calendering apparatus is used just to evaporate moisture within the towbands; only one third of the energy used in conventional calenders is actually used for elevating the filament temperature for heat setting purposes.
Not only does this expenditure of energy translate directly into increased cost, but the inefficiency of current calendering apparatus also limits production capability by limiting either the thickness of the towbands or the speed of tow travel to that which can be effectively heat set by a given calendering apparatus. Currently, the only way to achieve effective heat setting of higher density tows is to either increase the energy use by a calendering apparatus or increase the travel time of the tow through the calender apparatus.
While it is known to provide a squeeze roll at the entrance of a calendering apparatus to partially dewater the traveling towbands entering the calender, it is currently thought in the art that care must be used when subjecting synthetic filaments to pressure before the filaments are heat set in order to avoid damaging the filaments. For this reason, conventional squeeze rolls acting on synthetic filaments before heat setting are limited to lower nip pressures and made from resilient nip roll materials such as rubber in order to minimize the possibility of filament damage. For example, U.S. Pat. No. 4,112,668 to Spiller discloses a method for production of polyester staple yarn in which a tow is passed between a pair of squeeze rolls before being heat set and U.S. Pat. No. 3,968,571 to Oschatz et al. discloses a process of removing liquid from an absorbent substrate by passing the substrate through a pair of nip rollers, the surface of at least one of which is comprised of a sponge material having capillary pores. The nip roll arrangement disclosed in the Spiller patent is said to reduce moisture content of the tow below about 15%. The pressure at the nip between the rollers disclosed in the Oschatz patent is less than one kilogram per centimeter of roller length.
It is also known in the art that a high pressure nip roll mechanism may be used to remove moisture from a tow subsequent to heat setting of the tow filaments. In fact, high pressure nip rolls are conventionally used just prior to the crimping process to remove moisture and finish solvent applied to the tow before crimping. U.S. Pat. No. 4,197,622 to Williamson, for example, discloses a wet tow crimping process in which an advancing tow of fibers is uniformly compressed under a nip pressure of 600-1,000 pounds per inch to exude solvent-containing water from the tow just prior to crimping and U.S. Pat. No. 5,679,300 to Lorenz et al. discloses a method of treating a tow of melt-spun filaments in which the tow is passed through a pair of squeeze rolls after being heat set to reduce the fiber finish pickup to 0.7-7% by weight of the tow. As previously mentioned, however, it is generally thought in the art that such high nip pressures cannot be used prior to heat setting the filaments without sustaining damage to the filaments which would destroy them or at least render them unusable.
As shown by the above discussion, there exists a need in the art to effectively heat set synthetic tow filaments without expending a substantial amount of energy removing moisture from the tow before heat setting. This would allow less costly processing of synthetic filament tows and facilitate the processing of higher speed and more dense tows than is currently possible.